Welcome in this information page of Mppt Solar. On this page we will explain what an inverter, what is its function, what it consists of, what its working principle and what are the main types of inverters used in the most common situations and needs. With the help of simple diagrams illustrated, you will be guided in understanding this important electronic device essential in many fields of application. You will also find the wiring diagram to build a square wave inverter and lots of handy tips so you can make a purchase safe, aware and durable.

What is an inverter and what is this for. An inverter is an electronic device that can transform a direct current (DC) into alternating current (AC) at a given voltage and frequency. For example, if we have to feed a household appliance that operates in alternating current 230V (50Hz frequency) but we do not have available the alternating current network, thanks out occurs we can feed it, however, taking advantage of a direct current source, such as a 12V battery (DC) . E 'therefore essential to its use for supply by direct current, electrical devices that operate in alternating current. The inverters are used in photovoltaic off-grid (stand alone) for powering electric remote houses, mountain chalets, mobile homes, boats and are also used in grid-connected photovoltaic systems to enter the current produced by the plant directly into the power grid distribution (solar inverters). The inverters are also used in many other applications, ranging from UPS to speed controllers for electric motors, from power supplies switching to lighting. By the term "inverter" is designed to include a group "rectifier-inverter", supplied with alternating current and used to vary the voltage and the frequency of the alternating current output as a function of the incoming (eg for the supply of particular machinery). The most common inverters used to power the AC loads are of three types: square wave inverter (suitable for resistive loads), modified sine wave inverter (suitable for resistive, capacitive, inductive loads can produce noise) and pure sine wave inverter (suitable for all types of loads because faithfully reproduce a sine wave equal to that of our domestic power supply).

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How does an inverter works. Well, now we begin to explain this interesting phenomenon of transformation of energy. We have said that an inverter can obtain an alternating current from a direct current. To understand this phenomenon we should start with the explanation of an alternator. The alternator is a rotating electric machine that transforms the mechanical energy into electrical energy in the form of alternating current through the natural phenomenon of electromagnetic induction (an example is the alternator of the bike). In its simplest form, is composed of a coil of wire with a rotary magnet close to it. As soon as a pole of the magnet approaches the coil, it will create an induced current in the coil and this will flow in the opposite direction to the rotation of the magnet. Is then produced an alternating current.

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Now let's see how it behaves transformer. A transformer also produces an alternating current induced in the coil, but this time, the variable magnetic field is produced, not by a magnet but by another coil (called primary coil) having an alternating current flowing in it. Each coil traversed by an alternating electric current behaves like a magnet and produces a magnetic field. If the current direction changes, the polarity of the magnetic field changes.

Photo Credit: Solar-Facts

The useful thing is that of a transformer, the voltage produced in the secondary coil is not necessarily the same as that applied to the primary coil. If the secondary coil is composed of a double winding (has twice the number of revolutions) with respect to the primary coil, the secondary voltage will be twice the voltage applied to the primary coil. You can actually produce any voltage want varying the dimensions of the coils.

If in the primary coil, instead of alternating current, do scroll the DC current of a battery, is not to be formed no current induced in the secondary coil, because the magnetic field does not vary. But if we do change the direction of current continuously and quickly, then we have already made an inverter very simple and functional. This inverter produces as output a square wave, the frequency of which depends on the time in which we change the direction of the current circulating in the primary coil.

Photo Credit: Solar-Facts

How to make possible these continuous and rapid changes automatically? Using a transistor circuit, or even better achieved through MOSFET, or thyristors or IGBTs, that are more efficient. Below you will find the pattern to achieve a simple square wave inverter, using an astable multivibrator circuit for driving the primary coil. Of course this type of inverter is rich in harmonics and therefore is not suitable for food nor capacitive loads nor those inductive. It's possible to supply only purely resistive loads, such as filament lamps or electric heaters.

The transistors Q1 and Q2, as well as the transformer T1, determine how much power can supply the inverter. Q1 and Q2 are transistors 2N3055 and T1 is a transformer with maximum current 15A; in this case, the inverter can deliver about 300 watts. Remember that if you work with high currents, this inverter will absorb a considerable amount of current from the battery and in no time you could find yourself seriously damaged the battery. It 'well then prepare a sensor for the automatic interruption of the operation of the inverter as soon as the battery "drops" below a certain threshold voltage. It is also good to put a fuse, before the commissioning of the circuit.

Pure sine wave inverter. To obtain a sinusoidal alternating current at the output of our transformer, we must apply a sinusoidal current input. To produce a sine wave at the entrance of the primary coil we need an oscillator. One of the simplest oscillators we can achieve is surely a Wien Bridge with FET. The output is made stable thanks to the feedback.

Photo Credit: Solar-Facts

In most of the oscillator circuits, the output current will be of low intensity or in any case not sufficient to drive the main coil. This current will then need to be amplified from what will be more or less equivalent to a powerful audio amplifier so as to produce a high current to the primary coil of the transformer. The transformer, although very useful, it does not matter at all. While increasing the voltage, the current is reduced, and the power (voltage x current) remains the same (neglecting internal losses of the transformer). In other words, to obtain an output 1Kw alternating current, we provide incoming 1Kw DC.

The inverters best and most expensive are managed by a microcontroller and base their operation on the modulation of pulse width (PWM). The system can be fed back in order to provide a stable output voltage by varying the input voltage. For both types of modulation signal quality is determined by the number of bits employed. They range from a minimum of 3 bits to a maximum of 12 bits, able to describe with good approximation the sinusoid.

Grid Tied Inverter. A grid-tied inverters instead has a different function than the drive so far described. It in fact not only converts a direct current into alternating current, but is able to enter this current into the national grid. To perform this function, grid tied inverter must do the sampling of voltage and timing for the transfer.

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